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SI                               DENTITION OF SPARASSOBONTS                            55
Their correspondence with the milk series Is shown in an
interesting way by the close resemblance which the last milk
premolar often bears to the first molar. These two extremes of
dentition, i.e. purely monophyodont and, excepting for the molars,
purely diphyodont, are however connected by an intermediate state
of affairs, which is represented by more than one stage. In
JBorhyaena (probably a Sparassodont) the incisors and the canines
and two out of the four premolars belong to the permanent
dentition, while the two remaining premolars and of course the
three molars are of the milk series. Profhylcacinus, a genus
belonging to the same group, has a dentition which is a step or
two further advanced in the direction of the recent Marsupials.
"We nnd, according to Ameghino,1 whose conclusions are accepted
by !Mr. Lydekker, that the incisors, canines, and two premolars
belong to the milk series, while the permanent series is repre-
sented only by the two remaining premolars. "We can tabulate
this series as follows:
(1)  Purely monophyodont, with teeth only of the first set---
Toothed "Whales.
(2)  Incompletely monophyodont, as in the Marsupials, where
there is a milk dentition with only one tooth replaced.2
(3)  Incompletely  diphyodont,  with   the  dentition  made  up
partly of milk, partly of permanent teeth, as in Itorhyaena.
(4)  IKphyodont, where all  the  teeth  except   the  molars  are
of the second set;  this characterises nearly all the mammals.
As we pass from older forms to their more recent representa-
tives there is as a rule a progressive development of the form of
the teeth. This is especially marked among the Ungulata, The
extremely complicated type of tooth found in such a form as the
existing Horse can be traced back through a series of stages to a
tooth in which the crown is marked by a few separated tubercles
or cusps. Arrived at this point, the differences between the teeth
of ancestral Horses and ancestral ^Rhinoceroses and Tapirs are hard
to distinguish with accuracy; and the ^ame difficulty is experi-
enced in attempting to give a definition of other large orders by
the characters of the teeth, such as will apply to the Eocene or
1  Proc. Zool* Soc. 1899, p. 922.
2  Mr. M. "Woodward, however (P.Z.S. 1893, p. 467), is disposed to think that in
some Macropodidae at any rate the supposed tooth of the second set really belongs
to the milk dentition, arising late between Pra3 and Pm4.